The meeting was _____ at ten o’clock, but the chairman hadn’t turned up by eleven o’clock.

A．start

B．have started

C．be starting

D．be started

三阀组与（）送器配套使用，作用是将正、负压测量室与引压点导通或断开；或将正负压测量室断开或导通。

A、差压

B、温度

C、相对压力

D、绝对压力